Author: Sean Carroll

Our physics colloquium today was a departure; instead of a distinguished visitor telling us about forefront research, we had a talk by our own celebrated cosmologist Michael Turner. But he wasn’t talking about cosmology; for the last six months Michael has been in charge of the Mathematics and Physical Sciences Division at the National Science Foundation, and came back to tell us what life was like at the NSF.

At the end of his talk he left us with assignments: what tasks, in his opinion, were most important for the physics community at this moment. Number one in order of importance was to “broaden who we are,” by which he means to diversify away from domination by white males. To get an idea of the importance he was placing on this, the number two task was “do great science.”

Physics has been dominated by white men throughout its modern history. This fact doesn’t necessarily set it apart from other disciplines; but the depressing reality is that the situation in physics is improving only exceedingly slowly, if at all. Michael showed this picture of the University of Chicago physics faculty in 2000; more than thirty faces, none of them female. (At the time we actually had two women faculty, neither of whom happened to be present for the photo; now we have three, out of more than fifty faculty total.) We are not unrepresentative; less than ten percent of physics professors in the US are women, and it’s much worse at the senior level.

The graph shown on the right plots the percentage of women earning Ph.D.’s in selected fields, between 1980 and 1998. (Click the figure for more details.) It illustrates that the situation seems to be getting a little bit better, but also highlights how far we are from most other fields.

Why is it like that? I really don’t know. Anyone who has actually interacted with bright female physicists and students knows that the best women are just as good as the best men. There are also dramatic differences from country to country in the percentage of women in physics. So whatever the problem is, it’s not inevitable; there is something about our system that dissuades women from going into physics (and math, and engineering, and computer science).

My suspicion is that there is no one focused obstacle, and this is what makes the problem so hard to solve. Certainly there is sexism within the physics community, in all sorts of manifestations. I have seen straightforward examples of outright discrimination, where a male physicist would downgrade the abilities of a student or colleague simply because she was female; more commonly, a kind of unconscious sexism is at work, in which insecure men will simultaneously flirt (awkwardly) with women while not taking them seriously as researchers. This is the hardest to eradicate, since the perpetrators would never possibly accept that they weren’t extremely supportive of women in science. But in addition to direct sexism, there are elements of the scientific environment that are hostile, or even just uninteresting/unattractive, to female students, who subsequently leave the field of their own accord.

Unfortunately, the situation won’t be fixed by well-intentioned university departments aggressively pursuing the best women students or faculty (although they should). The problem begins back when children are very young, and girls are gently but persistently diverted away from science by a million subtle pressures. It might be that the only way to achieve gender equality in science is to completely overhaul the society, which strikes me as a big project (although worth undertaking).

Of course women are not the whole story when it comes to diversity; African-Americans, for example, are equally badly under-represented. But in that case the problem seems less subtle to me; it just doesn’t seem very surprising, since the economic conditions in which African-Americans grow up are often much worse than for whites, and the educations are correspondingly poorer. Physics, or academia more generally, is not a common career choice in families where it’s a struggle just to get a decent education. So to increase the representation of African-Americans in physics, all we have to do is to end economic inequality between the races in America. Easier diagnosed than accomplished, I suppose.

We all know what a mess the Administration got itself into by refusing to let Condoleeza Rice testify in public and under oath before the 9/11 commission, eventually being forced to give in. One of the main reasons their position was so silly is that Rice was constantly giving interviews to news shows at the very time she was refusing to testify in public and under oath; the obvious implication being that it was okay to talk, so long as you weren’t sworn to tell the truth, or if you were, so long as nobody would know what you said.

Now they are refusing to release the draft of the speech that Rice was scheduled to give precisely on September 11, 2001 — a major address outlining the administrations foreign-policy strategies. We all know why they wouldn’t want it made public; the painful truth that the administration was focused on state-based threats and swooning over missile defense systems, when they should have been concentrating on asymmetric threats from terrorist organizations, would be glaringly obvious. (See Josh Marshall’s post from last week, based on excerpts from the speech.) But do they really not see how dumb this looks? How secret can it be? It was a speech she was going to deliver! One presumes there would have been people in the room, listening to it and stuff.

Update: Tbogg says much the same thing. Not that I thought this point was so subtle you had to be an overeducated theoretical physicist to figure it out or anything.

And now I’m on the road again (giving a colloquium in Rochester), so still no time for a substantive post. But I did find out that the pie chart that serves as the unofficial logo of this blog will appear on the nametags and program for this conference next week. Not because of the blog (I presume), but because it represents our inventory of the universe: ordinary matter, dark matter, dark energy. Representing the inventory as a pie chart is almost too obvious, but people sometimes ruin the idea by making it too complicated. The universe is actually a simple place, although a subtle one.

There really should be a way to make money off of this blogging thing. Perhaps Preposterous Universe coffee mugs with the pie-chart logo? Is it possible to finance a new car this way?

I would love to write my promised post on quantum gravity, but duty interferes, and I need instead to write a lecture on special relativity for class. Instead, here are two interesting facts about Hendrick Antoon Lorentz, one of the founding fathers of relativity.

First, although he was the inventor of Lorentz transformations, he went to his grave (1928) not believing in Lorentz invariance! He thought his transformations were just a trick for transforming between inertial frames and the one true ether frame. Einstein figured out in 1905 that the ether frame was unnecessary, and that’s when special relativity really got off the ground. For more info, look at the bottom of this page for Michel Janssen’s dissertation.

Second, he did not invent the Lorenz gauge of electromagnetism (note the spelling). That would be Ludwig Lorenz, a Danish physicist. Poor Ludwig’s reputation was lost in the glow of someone with awfully similar name and interests.

The most recent issue of The New Republic has an article about the Pledge of Allegiance affair by Leon Wieseltier. It’s an insightful piece — Wieseltier, who seems to be religious himself, puts the issue in better perspective than I ever could have. His main point is simply that the defenders of keeping “Under God” in the Pledge are actually undermining religion, since their main tactic is to claim that the phrase doesn’t really refer to anything specific, just a warm and fuzzy feeling we all have as Americans. Wieseltier correctly points out that it is the atheists who, by not buying into such a meaningless notion of God and religion, are the ones who take God seriously.

For this reason, American unbelief can perform a great quickening service to American belief. It can shake American religion loose from its cheerful indifference to the inquiry about truth. It can remind it that religion is not only a way of life but also a worldview. It can provoke it into remembering its reasons. For the argument that a reference to God is not a reference to God is a sign that American religion is forgetting its reasons. The need of so many American believers to have government endorse their belief is thoroughly abject. How strong, and how wise, is a faith that needs to see God’s name wherever it looks?

I think he’s exactly right — religion only makes sense if it pleads guilty to making claims about how the world works. I also believe that those claims fall far short, but I have more respect for believers who stand by the manifest consequences of their belief.

Just so we don’t get too uppity, keep in mind that the subject matter of the previous two posts might be completely discredited within a few years, due to the ceaseless efforts of the supernatural movement. (Link from Panda’s Thumb.)

Wow, with a title like that, I should just end the post right now; it’s going to be all downhill from there. You could make a lot of money from a book with a title like that. Or did Deepak Chopra already write it?

Okay, I’m delaying the inevitable. I was hoping today to write about quantum gravity, after once and for all explaining the mysteries of quantum mechanics in the previous post. But I carelessly brought up the issue of the interpretation of the theory, which deserves more nuanced discussion. Not that I’m qualified to give it. You can read something about the issues at Michael Nielsen’s blog (two posts).

But I would like to at least say some words about what I think the issue is, even if I don’t want to make a strong case for any particular resolution. There certainly is an “issue,” which may or may not be a “problem.”

Every scientific theory comes in two pieces: a formal structure, and an “interpretation” that maps this structure onto what we see. Usually the interpretation is perfectly obvious, and we don’t worry about it. But in quantum mechanics, what we see is not what there is, so we need to think more deeply. What is it that really happens when we do a measurement? For example, consider an unstable nucleus. Its wavefunction is a combination of two classical possibilities: the nucleus has already decayed, or it hasn’t. But when we observe it, we don’t see this superposition of possibilities; we see that it has either decayed or not. What really happens when we look at it? The Copenhagen interpretation says that the wavefunction collapses to the possibility that we have observed, while the many-worlds interpretation says that the observer+nucleus system evolves smoothly to a superposition of “nucleus decayed, observer saw decay” and “nucleus intact, observer did not see decay.”

The MW interpretation is nice because everything is smooth evolution obeying the laws of physics (in this case the Schrodinger equation). But it’s tricky because, since “I” actually do or do not see the nucleus decay, I need to identify “I” with a certain “branch of the wavefunction,” not with the entire wavefunction. This is hard to do, both technically and conceptually. (“What does “I” mean? How does this branching process take place?)

I’m in the camp that says it’s fair to call this a philosophy problem, not a physics problem. But it’s a perfectly legitimate philosophy problem, not a silly waste of time. Fortunately for physicists, we don’t need to know the answer to make progress on the questions we really care about. (Apparently, anyway; statements like that have a way of showing up in future textbooks as evidence of how misguided past generations were.)

The joint is jumping over at Peter Woit’s Blog, even though his is even newer than mine. (Blog statistics are just what academia needs: another quantitatively precise and wholly meaningless measure of worth.) I suspect it’s because Peter tends to say provocative and controversial things that people readily disagree with (about emotional topics like, say, string theory), whereas I am so sweetly reasonable that everyone cannot help but agree with everything I say.

Peter did ask a question to cosmologists that I didn’t get to, so I thought I should take a swing: “What is ‘string cosmology’?” If the response were to make any sense, I should explain something about string theory, which means explaining something about quantum gravity, which means explaining something about ‘quantum’ even without gravity. I don’t know how far we’ll get, but explaining quantum mechanics is a worthy goal in its own right.

Quantum mechanics (QM) is one of the top two most profound ideas in the history of physics. The other member of the top two is classical mechanics, the system developed by Galileo and Newton and their friends, which was eventually superseded by quantum mechanics. (The ordering of the top two is tricky, and there’s no consensus on number three.) Nevertheless, QM is consistently misrepresented (or even misunderstood) by professional physicists, and its basic ideas aren’t nearly as clear to people on the street as they should be.

Classical mechanics is simple. For any physical system (balls on a billiard table, planets moving around the sun, the whole universe) you tell me the “state” that the system is in at some time, and I can use the laws of physics to predict what the state will be at any other time. Specifying the state typically means specifying the positions and velocities of all the components. This kind of system is at the heart of the “clockwork universe” that came out of the Enlightenment.

Quantum mechanics came about in the early 20th century. Surprisingly, the description of classical mechanics in the previous paragraph also applies perfectly well to quantum mechanics: you tell me the state, I’ll use the laws of physics to evolve it forward in time (or backward, for that matter). The crucial difference lies in a feature so profound that it’s hard to conceptualize: in quantum mechanics, what you can see (the observable properties of the system) is related to, but not the same as, what there really is.

So, for a single particle, classical mechanics tells us that it has a position and a velocity. The lesson of quantum mechanics is sometimes garbled into the idea that “we can’t be perfectly certain where the particle is or how fast it is moving.” The truth is more profound: there is no such thing as “where the particle is,” or “how fast it is moving.” Instead, there is something called the wavefunction that describes the state of the system. The wavefunction answers the question, “when we observe the system, what is the probability we will observe it to have a given position or velocity?” In classical mechanics we can observe anything we want about the state, but in quantum mechanics we can’t, we can only predict probabilities for what might happen when we make an observation.

What actually happens when we make an observation is the source of great philosophical angst. The old “Copenhagen interpretation” held that the wave function changed instantaneously and non-locally, into a state that was concentrated around the result of our observation. The newer (but still pretty venerable) “many-worlds interpretation” says that we the observers are also described by wave functions, and the measurement process mixes up our wave function with that of the thing we’re looking at in such a way that we only ever experience unique outcomes for observations, even though everything is evolving smoothly. As crazy as it sounds, many working physicists buy into the many-worlds theory (and, like approval for gay marriage, there is a significant demographic slant, in which younger people are more open).

Quantum mechanics is not so much a theory as it is a framework in which we can propose all sorts of specific theories. The most empirically successful are quantum field theories, in which the elements of our physical reality are fields defined on spacetime (quantities that take on values at every point, like an electric field). In quantum field theories, the actual field values are one of these unobservable things; what we can actually see is discrete excitations of the fields that we call “particles.” Quantum field theory successfully describes every experiment ever performed and every phenomenon ever observed, with one glaring exception: gravity. For a force that is so important, it’s truly embarrassing that we can’t fit it into our favorite framework. That’s why so many physicists think that the search for a consistent quantum theory of gravity is so interesting and vital.

P.S. (When reading Peter’s most recent post, please keep in mind the date posted.)

This quarter I don’t get to moonlight in the humanities; I actually have to teach a physics course. But it’s a fun one: Spacetime and Black Holes, an introduction to general relativity for undergraduates. GR is Einstein’s theory of gravity; it can be summed up in the simple statement “Gravity is the curvature of spacetime.” It plays a crucial role in understanding black holes and neutron stars, the big bang and the accelerating universe, gravitational waves, and every attempt to quantize gravity.

Teaching GR to undergraduates is still unusual; at many places it isn’t even a core graduate course. (Of course, these days they’re teaching undergraduates string theory.) For a long time GR was somewhat outside the main action of physics, since our experiments didn’t probe into regimes where it was important. That’s certainly changed in recent years. GR also has something of a reputation for being difficult, which is quite untrue; it’s intrinsically very straightforward, but the relevant mathematics (tensor analysis, differential geometry) is just so different than that used in other areas of physics that it seems like a big investment to learn.

This quarter I’ll be using Jim Hartle’s new book, which is a fantastically useful text. He approaches the subject with a physics-first attitude that allows the student to get to the fun parts without spending months learning formalism. (If they want to do that, they should take the graduate course and buy my book.) We just state without demonstration what the spacetime around a star or black hole looks like, and then dive right in to understanding its features. I’ve never actually taught it this way before, so it’s something of an experiment. The worry is that the students will fear that they’re getting a watered-down version of the true story, which really isn’t the case. By the end they’ll get the whole shootin’ match. If I would just quit blogging and write my lecture, anyway.

The Bush administration is zealous about so many nutty things it’s hard to keep track. Missile defense (“Star Wars” and its ilk) is one of them I had almost forgotten about. Apparently they hope to spend over $10 billion per year to develop a defense that doesn’t work against an enemy that doesn’t really exist.

Physicists have long known that the missile-defense plans are mostly scams; they are wildly optimistic, overhyped, undertested, and usually misrepresented. It’s just hard to shoot down a bullet with another bullet; and when the incoming bullet can use countermeasures, it’s practically impossible. Now a group of generals and admirals is saying the same thing: this is a colossal waste of money, let’s spend that money doing something useful like, say, protecting against terrorism. The military experts have an uphill battle; they don’t appreciate that the administration finds the battle against terror kind of boring, and is easily distracted by shiny objects.